1. Field of the Invention
The present invention relates to improvement of an image forming device having a two sided copying capability in a copier, a printer, a facsimile device, etc., which is capable of forming a toner image on both surfaces of the recording paper by use of an electro-photographic method.
2. Description of the Related Art
An image forming device utilizing an electro-photographic process in conventional devices such as a copying machine, a printer, a facsimile device or the like is provided with a recording paper feeding apparatus. The recording paper fed by the paper feeding apparatus is transported into an image forming portion having a photosensitive body at the center thereof, and an image is formed on the transported paper. The recording paper supplied to the paper feeding apparatus is usually blank and does not have an image on either side thereof. However, it is sometimes necessary to write in an image by use of the above-mentioned image forming apparatus on specially-used recording paper having a predetermined format consisting of characters, lines or figures already printed thereon.
However, there exists hitherto no image forming apparatus specifically designed for using recording paper already copied on one surface thereof and accommodated in the paper feeding cassette or tray, and thereafter forming an image on the other surface which is blank.
There presently exists a conventional device capable of automatically detecting the size of the recording paper accommodated in the paper feeding apparatus. However, there exists no device capable of judging whether only one surface of the recording paper has already been printed on.
There also exists hitherto a device capable of judging whether the size of recording paper selected in a both-sides copying mode is capable of being accommodated in the paper-refeeding apparatus, stopping the operation of the mode in case that the selected size cannot be accommodated in the paper-refeeding apparatus, and giving a warning to the operator.
For instance, Japanese Laid-Open Patent publication No. 58-40564/1983 discloses a device comprising first and second sheet accommodating portions and constructed so as to detect the size of the sheet in one sheet accommodating portion when no sheet is in the other sheet accommodating portion, and to change over the sheet accommodating portion and feed the sheet of paper to the accommodating portion for which the size of the sheet is fitted.
Furthermore, Japanese Laid-Open Patent publication No. 63-87429/1988 discloses a device capable of performing a copying operation and emitting an alarm in case recording paper of a size which cannot be accommodated in the paper refeeding apparatus is selected when a copy-mode which requires accommodating the recording paper in the paper refeeding apparatus is selected.
However, in all of the above-mentioned cases, there are only performed processes of control and display in accordance with the size of the recording sheet accommodated in the paper feeding apparatus. There is neither the function of detecting and judging whether one surface of the recording paper has already been printed or both surfaces of the paper are blank, nor the function of causing the device to perform processes of control and display in accordance with the result of the above judgement.
In recent years an increasing number of users perform image formation on the rear surface of recording paper, one surface of which already has an image formed thereon, in order to realize savings of resources and expenses. Furthermore, both-sides copying has often been done for the same reason. Therefore, in case that the user selectively executes one of two modes, a mode of copying on the rear surface of recording paper which has been already copied on its one surface or another mode of both-sides copying, it was necessary to confirm each time the sort of the recording paper accommodated in the paper feeding apparatus, i.e., whether both surfaces of the paper were blank or one surface of the paper had been already copied, and to select the paper feeding cassette which accommodated recording paper in accordance with the selected mode.